Adjustable anti-tip wheels for power wheelchair

ABSTRACT

A power wheelchair is provided having one or more anti-tip wheels for stabilizing the movement of the frame about the drive wheels. An anti-tip wheel is attached to the end of a suspension arm which is pivotally attached to the frame of the wheelchair. A spring resiliently supports the anti-tip wheel in a neutral position relative to the ground upon which the drive wheels of the wheelchair normal ride. A retaining system selectively fixes position of the anti-tip wheels relative to the frame. The retaining system employs a plurality of teeth, with one portion disposed in combination with said suspension arm, and at least one other tooth disposed for engaging said one portion to provide unidirectional retention of the position of said suspension arm relative to the frame. The engagement of the teeth to lock the position of the suspension arm may be responsive to the torque generated by the motor.

RELATED APPLICATIONS

The present application is a continuation of co-pending application Ser.No. 10/139,191, which issued as U.S. Pat. No. 6,923,278 on Aug. 2, 2005.

BACKGROUND OF THE INVENTION

The invention is directed to wheelchairs, and more particularly to frontanti-tip wheels for power wheelchairs. However, various aspects of theinvention may be used in other applications.

Power wheelchairs are known and have been the subject of increasingdevelopment efforts to provide disabled persons with independentmobility. Early power wheelchairs resemble conventional wheelchairs,having a drive wheel at the back of the chair. One difficulty associatedwith rear wheel drive wheelchairs is that they typically have very wideturning radiuses. The front or mid-wheel power wheelchair is animprovement over these early power wheelchairs. Among other advantages,front or mid-wheel power wheelchairs have tighter turning radiuses. Bothfront and mid-wheel drive wheelchairs will be hence forth referred togenerally as mid-wheel power wheelchairs. Many improvements uponconventional mid-wheel power wheelchairs have been made. U.S. Pat. No.5,944,131 to Schaffner et al., which is incorporated herein, disclosesan improved mid-wheel power wheelchair.

One of the difficulties with mid-wheel power wheelchairs is the tendencyto tip in a forward attitude during a sudden stop or while driving downan incline. In response to this problem, anti-tip wheels have been addedto the front of power wheelchairs. One mid-wheel power wheelchairembodying front anti-tip wheels is disclosed in U.S. Pat. No. 5,848,658to Pulver. The anti-tip wheels typically are attached to an arm orsuspension system which suspends the anti-tip wheels some distance abovethe ground. The suspension of the wheels above the ground is necessaryso that the wheelchair can clear small obstacles in its path of traveland turn without skidding the wheels sideways. This configuration,however, allows the wheelchair to tip slightly in a forward attitudebefore the anti-tip wheels engage the ground. Although the anti-tipwheels prevent the wheelchair from tipping over fully, the conventionalsystem allows a partial tip of the wheelchair when a sudden stop occursor while traveling down an incline. Such a partial tip is a highlyuncomfortable and upsetting experience for the user. The presentinvention solves the problem of partial tipping of the wheelchair.

SUMMARY OF THE INVENTION

The present invention is directed to a wheelchair having one or moreanti-tip wheels and a supporting structure for maintaining the anti-tipwheels in a position very near the ground when desired, therebyeliminating the uncomfortable sensation of a partial tip for the user. Alowering mechanism is provided to place the anti-tip wheels in the nearground position. The lowering mechanism may also act as a retainingsystem or a separate system may be provided for holding the anti-tipwheels in the near ground position. A quick release mechanism is alsoprovided for releasing the anti-tip wheels from the near ground positionwhen an obstacle in the path of travel is encountered.

Preferably, the anti-tip wheels are mounted on an arm extendingforwardly from the frame of the wheelchair. The arm is provided with astrut connected to a spring near the middle of the arm, the spring beingattached to the frame of the wheelchair at a location on the framehigher and forward of the attachment point between the frame and therear of the arm. When an obstacle is encountered in the travel path ofthe wheelchair and the retaining system is disengaged, the anti-tipwheels are forced upwardly, thereby compressing the spring and allowingthe wheelchair to ride over the object.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is a right side view of a wheelchair according to a firstembodiment of the invention with part of the drive wheel cut away.

FIG. 2 a is a side view of an anti-tip wheels assembly according to thefirst embodiment with anti-tip wheels in a neutral position.

FIG. 2 b is a view of the assembly shown in FIG. 2 a with anti-tipwheels pushed upwardly by contact with an obstacle.

FIG. 2 c is a view of the assembly shown in FIG. 2 a with its anti-tipwheels engaged in a near ground position.

FIG. 3 a is a perspective view of the motor and cam assembly of theembodiment of FIG. 2 with the anti-tip wheels in a neutral position. Aportion of the frame above the motor and lower portions of the springsare cut away.

FIG. 3 b is a perspective view of the motor and cam assembly of FIG. 3 awith the anti-tip wheels in a near ground position. The lower portionsof the springs are cut away.

FIG. 4 a is a broken schematic side view of a second embodiment of theinvention with anti-tip wheels in a neutral position.

FIG. 4 b is a broken schematic side view of the second embodiment of theinvention with anti-tip wheels in a near ground position.

FIG. 4 c is a broken schematic side view of the second embodiment of theinvention with anti-tip wheels in a near ground position.

FIG. 5 is a perspective view of a retaining system according to theembodiment shown in FIG. 4.

FIG. 6 is an exploded view of a ratchet, gear and gear shaft assembly ofthe embodiment of FIG. 5.

FIG. 7 is a schematic diagram of a control circuit for controlling ananti-tip wheel motor.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like numerals represent likeelements, FIG. 1 shows a mid-wheel power wheelchair, according to afirst embodiment of the invention, generally designated by the numeral10. Although the invention may be practiced with only one anti-tipwheel, the invention will sometimes be described with reference topreferred embodiments, which have two anti-tip wheels. Wheelchair 10comprises front anti-tip wheels assembly 20 connected to at least twopoints on frame 60. One experienced in the art will recognize that theinvention could be used toward the rear of the wheelchair to preventtipping during a sudden acceleration or travel up an incline. However,the description provided herein focuses on its use at the forward end ofthe wheelchair. Anti-tip wheels assembly 20 will be described in greaterdetail below.

Wheelchair 10 further comprises drive wheels 62 which are rotatablyconnected to frame 60 and driven by a single drive motor or areindependently driven by dual drive motors (not shown). Suitable drivemotors for use in mid-wheel power wheelchairs, as well as suitablemounting systems, differentials, and axle systems for connecting to thedrive wheels, are well known to those skilled in the art. Rear wheel 64is shown as a castor wheel connected to frame 60, it being understood,however, that the rear wheel 64 need not be a castor wheel. Rather, rearwheel 64 may take many alternative forms, including the form of anti-tipwheels assembly 20.

Wheelchair 10 further comprises a seat 66 which is preferably removablyattached to frame 60. A back rest 68 may be attached to seat 66 or frame60. An arm rest 70, having a control stick 72 is also attached to frame60.

Referring now to FIGS. 2 a, 2 b, 2 c, 3 a and 3 b, anti-tip wheelsassembly 20, should be connected to frame 60 in at least two places,whether directly or indirectly. An arm 24 comprises front portion 26 andrear portion 28, which are rigidly connected to one another to form theforwardly extending arm 24. Rear portion 28 is pivotally connected tothe frame by a bolt 32. A wheel 22 is rotatably connected to the frontportion 26 by a pin 30. A cam follower 36 extends upwardly from the rearportion 28 of arm 24. Arm 24 is further connected to a dampening devicecomprising strut 38 and spring 40, which extend upwardly therefrom.Strut 38 and spring 40 are pivotally connected to arm 24 via a pin 34.

The connection of strut 38 to pin 34 and arm 24 can best be seen inFIGS. 3 a and 3 b, which shows this embodiment having two anti-tipwheels and lowering mechanisms to engage each. Each homologous elementlooks and functions the same, therefore each element will be describedin the singular to avoid confusion. The connection of spring 40 to pin34 and arm 24 is not shown, the lower portion of spring 40 being cutaway to reveal strut 38. It should, however, be clear that spring 40 ispivotally connected with pin 34 in a similar fashion as strut 38. Spring40 is coiled about strut 38 and both are engaged with support member 74at their upper ends. Support member 74 is rigidly attached to, and formsa part of frame 60, preferably at a point higher than the point at whichrear portion 28 of arm 24 is attached to the frame. Spring 40 is fixedlyattached to support member 74, while strut 38 is slidingly engaged withsupport member 74 such that the upper end of strut 38 may slidetherethrough when spring 40 is compressed.

Anti-tip wheels assembly 20 further comprises a mechanism for loweringarm 24 and wheel 22. The lowering mechanism must be capable of forcingdown arm 24 and wheel 22 out of a neutral position (FIG. 2A, describedbelow) where spring 40 tends to hold them. In this embodiment, thelowering mechanism is capable of holding wheel 22 in a near groundposition (FIG. 2C, described below). As used herein, near groundposition is understood to include a ground engaging position wherein thewheel 22 is in contact with the ground. Such a ground engaging positionis desirable if the user is traveling for a substantial distance on asmooth surface without turning or stopping. Otherwise, the near groundposition is more preferably a distance off the ground which allows thewheelchair 10 to turn without wheel 22 catching awkwardly or skiddingsideways, yet not so far above the ground, as to allow wheelchair 10 totip significantly. Such a position may be only a fraction of an inchabove the ground when the wheelchair 10 is used on a smooth surface,such as an indoor floor. The near ground position may be slightly higherwhen wheelchair 10 is to be used on rougher terrain.

In this embodiment, the lowering mechanism is capable of holding the arm24 and wheel 22 in the near ground position, thereby acting also as aretaining mechanism. The lowering mechanism may be released from itsengagement with arm 24 by rotating the anti-tip wheel motor 50 in an updirection or more quickly by disengaging the motor such as bydisengaging gears within the gear box 52, which is described immediatelybelow. Releasing the lowering mechanism allows the spring 40 to tend toreturn arm 24 and wheel 22 to the neutral position.

The lowering mechanism of the first embodiment can best be understood byexamining FIGS. 3 a and 3 b together with FIGS. 2 a, 2 b, and 2 c. Thelowering mechanism comprises an anti-tip wheel motor 50 preferablymounted to the frame 60. As seen in FIG. 3 a, where part of the frame 60is cut away, the anti-tip wheel motor 50 is connected to a gear box 52and provides power thereto when activated by the user. Cam shaft 54 isrotatably connected to gear box 52 and receives rotational powertherefrom. Two cams 56 a and 56 b are provided as seen in FIGS. 3 a and3 b. In this embodiment, cam shaft 54 preferably serves as a connectingrod between the cams 56 a and 56 b. Otherwise, two motors may power twocam shafts which independently rotate two cams. The cams 56 a and 56 bare rigidly mounted to both ends of cam shaft 54. In this form of theinvention, the cams 56 a and 56 b are in rotational unison. As shown inFIGS. 2 a, 2 b, and 2 c, in which cam 56 a is identified simply by thenumber 56 and represents the function of both cams, cams 56 must bemounted such that they engage followers 36 when rotating downwardly. Camshaft 54 may be removably mounted to the frame 60. The removablemounting is desirable if the wheelchair battery (not shown) is frontremovable.

Referring specifically to FIG. 2 a, cam 56 is shown in its up position.The cam 56 is therefore exerting no force on follower 36. Thus, arm 24is free of any influence from the lowering mechanism. The forces actingon the arm 24 in this state are the downward gravitational force and theforce of the spring 40 tending to assume its relaxed state, holding thearm 24 and wheel 22 in the neutral position. Arm 24 and wheel 22 arefree to move up or down, as far as spring 40 will tolerate under anyparticular external force. In this state, anti-tip wheels 22 will risewhen they encounter an upward force, such as when the wheelchair 10encounters a curb. Also in this state, strut 38 extends partiallythrough the top of support member 74.

It should be clear to those of ordinary skill in the art, that theposition of the anti-tip wheels 22, in terms of their height off theground, while free of influence from the lowering mechanism or externalforces, is dependent on the size and tension of the spring 40. The sizeand tension of the spring 40 may therefore be preselected and may dependupon the size of the user as well as the user's preferences concerningthe height of the wheels 22 off the ground.

FIG. 2 b shows the state of the anti-tip wheels assembly 20 when thecams 56 are in an upward position and an upward force is applied to thewheels 22. As in FIG. 2 a, the lowering mechanism exerts no force on arm24. An upward force may be generated when wheel 22 encounters anobstacle, such as a curb 100. Spring 40 resists the upward force; butwhere the force is great enough, spring 40 becomes compressed, allowingarm 24 and wheel 22 to rise. Strut 38 is free to slide upward throughsupport member 74. While raised, wheel 22 is able to clear obstacle 100.

FIG. 2 c shows the anti-tip wheels assembly 20 with the loweringmechanism engaged. Cam 56 is in its downward position engaging follower36. Spring 40 is stretched beyond its relaxed state. The spring resiststhis state, tending to return to the neutral position shown in FIG. 2 a.However, the force exerted on arm 24 via cam 56 and follower 36 is greatenough to overcome the resistance of the spring 40. As long as cam 56remains in the downward position, arm 24 is forced to remain downdespite the tendency of spring 40, thereby holding wheel 22 in aposition very near to the ground (the near ground position). Thus, thecam 56 acts as both the lowering mechanism and the retaining system,thereby providing a unified anti-tip wheel positioning assembly.

As shown in FIG. 3 b, followers 36 each preferably include a roller 44rotatably connected to the upwardly extending portion of follower 36 forengaging cam 56. Where the rollers 44 are not employed, cams 56 mayengage followers 36 directly. The use of rollers 44 allows thetransmission of power from the cams 56 to the followers 36 to occursmoothly. Further, the rollers 44 will sustain less wear than a flat orknife type follower, thereby requiring less maintenance and preventingreplacement after extended use.

As discussed above, and shown in FIGS. 3 a and 3 b, the power for thelowering mechanism is provided by anti-tip wheel motor 50 which shouldbe connected to a power source (not shown) through wires 76. Many typesof motors are suitable for providing the power for the loweringmechanism. The preferable motor speed is from about 8 to about 30revolutions per minute.

Control circuits are preferably provided to control the motion of theanti-tip wheel motor 50 and prevent back-drive thereof. Preferably, thecontrol circuits include an up control circuit for operating theanti-tip wheel motor in an up direction and a down control circuit foroperating the anti-tip wheel motor in a down direction. In addition, asafety interlock control circuit operates to prevent any drive voltagefrom operating the anti-tip wheel motor under predetermined conditions,such as when neither the up nor down control circuits are operating orwhen a user inadvertently operates both the up and down circuitssimultaneously. Further, up and down limit control circuits stop theanti-tip wheel motor at respective predetermined limits of upward anddownward movement of the anti-tip wheels.

The up and down control circuits each include manual control elements,such as user-actuated switches, in electrical communication with avoltage source. The manual control elements may be distinct up and downdirection switches, or elements of a three-position (or more) switch,which serves both capacities. For purposes of clarity, only anembodiment of the circuit with separate up and down switches will bedescribed in detail. Each up and down circuit also includes a time delaycircuit element, and a switching circuit to energize the anti-tip wheelmotor respectively in the up or down direction.

The safety interlock control circuit includes a logic gate having aninput connected to each of the up and down switches, and a switchingcircuit to create a short circuit across the anti-tip wheel motor.

The up and down limit control circuits each include an automatic controlelement that is actuated at the respective predetermined limits ofupward movement of the anti-tip wheels, and a disabling circuit thatdisables the respective switching circuits of the up and down controlcircuits.

A first relay is in electrical communication with the up control switchacross a capacitor and through one or more components causing apropagation delay, i.e., a time delay element. In use, the first relaysupplies a voltage to a first side of the motor 50 when the motorcontrol switch is closed. Further, a transistor is in electricalcommunication with the up switch through a logical inverter. Thetransistor is connected to the capacitor and discharges the capacitorwhen the motor control switch is open.

When neither the up nor down switches are closed, first logical signalsare provided to the logic gate, which produces a first output. Theoutput of the logic gate is connected to a second relay, which closes anelectrical connection across the anti-tip wheel motor 50 when the logicgate provides the first output. When either of the manual controlelements is closed, a second logical input is provided to the logicgate, driving the logic gate to produce a second logical output. Thesecond relay opens the electrical connection across the motor 50 whenthe logic gate provides a second output.

The down switch is connected to and acts on a third relay in ananalogous manner as the up switch and first relay. The third relay is inelectrical communication with the down direction switch across a secondcapacitor and through a second component having a propagation delay. Thethird relay supplies a voltage to a second side of the anti-tip wheelmotor when the down direction switch is actuated. A second transistor isin electrical communication with the down direction switch through asecond logical inverter. The second transistor is connected to thesecond capacitor and discharges the second capacitor when the downdirection switch is open.

The up and down limit circuits, stop the motor when the anti-tip wheelsmove to the respective predetermined heights. Each limit switch isautomatically actuated (opens) when the anti-tip wheels reach thepredetermined height. The limit switches each connect a first heightlimit transistor to a first height limit voltage source when closed.When each limit switch is closed, the first height limit transistordraws voltage from a second height limit voltage source. When the heightlimit switch is open, the second height limit voltage source biases asecond height limit transistor, which then draws voltage from the inputof the logic gate and the first or third relays, respectively, turningoff the motor 50.

A preferred embodiment of the control circuit will now be described ingreater detail. Up and down switches 202, 204, which are shownschematically in FIG. 7, are connected to a 5 volt power source. Thecontrol circuit is shown with both the up and down switches 202, 204open, which is their default position while the motor 50 is in a standbystate. The up switch 202 provides a first input to an exclusive NOR gate206 via a 1 K (Kilohm) resistor 208 and diode 210. The first input isgrounded through a 51 K resistor 211. The down switch provides a secondinput to the exclusive NOR gate 206 via a 1 K resistor 212 and a diode214. Similar to the first input, the second input to the exclusive NORgate 206 is grounded through a 51 K resistor 215. While both switchesare open, both inputs to the exclusive NOR gate 206 are low, producing ahigh output from exclusive NOR gate 206, which charges a 10 microfaradcapacitor 216 through a 10 K resistor 218. With the capacitor 216charged, a bipolar transistor 220 becomes biased through a diode 222 and1 K resistor 224. The connection between resistor 224 and transistor 220is grounded through a 51 K resistor 225. The biasing of transistor 220causes a relay K2 (element 226) to toggle, which creates a short acrossthe motor 50.

When the up switch 202 is pressed, a logical high is provided to thefirst input at the exclusive NOR gate 206 via resistor 208 and diode210. The output of the exclusive NOR gate becomes low, causing capacitor216 to discharge, transistor 220 to unbias, and relay K2 to toggle,thereby opening the short across motor 50. In addition, a 47 microfaradcapacitor 228 is charged through a diode 230 and 33 K resistor 232. Alogical high is thereby produced at the output of a pair of inverters234 and 236, which biases a transistor 238 through a 1 K resistor 240.The connection between transistor 238 and resistor 240 is grounded via a51 K resistor 241. Biasing the transistor 238 toggles a second relay K1(element 242), which causes 24 volts to be supplied to the motor 50,driving the motor to turn in the upward direction. Charging thecapacitor 228 through the 33 K resistor 232 creates a short time delayand the inverters 234, 236 create a small propagation delay in togglingthe relay K1. It is thereby ensured that the short across the motor hasbeen opened by relay K2 before power is supplied to the motor 50 byrelay K1.

Also while the up switch 202 is closed, a logical low is provided totransistor 244 through an inverter 246 and 1 K resistor 248. Theconnection between the transistor 244 and resistor 248 is grounded via a51 K resistor 249. With the logical low input, the transistor 244 isunbiased and does not conduct. When the up switch 202 is opened, theinverter 246 provides a logical high to bias transistor 244, whichdischarges capacitor 228. The relay K1 is thereby toggled open, quicklystopping the motor 50.

When the down switch 204 is closed, a logical high is provided to thesecond input at the exclusive NOR gate 206 via resistor 212 and diode214. The output of the exclusive NOR gate becomes low, causing capacitor216 to discharge, transistor 220 to unbias, and relay K2 to toggle,thereby opening the short circuit across the motor 50. In addition, a 47microfarad capacitor 250 is charged through a diode 252 and 33 Kresistor 254. A logical high is thereby produced at the output of a pairof inverters 256 and 258, which biases transistor 260 through a 1 Kresistor 262. The connection between transistor 260 and resistor 262 isgrounded via a 51 K resistor 263. Biasing the transistor 260 toggles athird relay K3 (element 264), which causes 24 volts to be supplied tothe motor 50, driving the motor to turn in the downward direction.Charging the capacitor 250 through the 33 K resistor 254 creates a shorttime delay and the inverters 256, 258 create a small propagation delayin toggling the relay K3, thus ensuring that the short across the motorhas been opened by relay K2 before power is supplied to the motor 50.

Also while the down switch 204 is closed, a logical low is provided totransistor 266 via an inverter 268 and 1 K resistor 270. The connectionbetween the transistor 266 and resistor 270 is grounded via a 51 Kresistor 271. With the logical low input, the transistor 266 is unbiasedand does not conduct. When the down switch 204 is released, the inverter268 provides a logical high to bias transistor 266, which dischargescapacitor 250. The relay K3 is thereby toggled off, quickly stoppingdownward motion of the motor 50.

Up and down limit switches are also provided to stop motion of the motorwhen the anti-tip wheels reach a pre-selected up or down position. Theup-limit switch 272, which is connected to a 5 volt source 271, isnormally closed and connected to a transistor 274 through a 1 K resistor276. The connection between the transistor 274 and resistor 276 isgrounded via a resistor 277. While the up-limit switch 272 is closed,transistor 274 is “on”, draining another 5 volt source 278 connectedthereto through a 1 K resistor 280. That keeps a second transistor 284unbiased and “off”. The up-limit switch 272 opens when the anti-tipwheels travel to their highest allowed point off the ground. Theup-limit switch 272 can be opened mechanically by the wheel arm 24, orvia a relay connected to a remote sensing device (not shown). When theup-limit switch 272 is open, the connection to the voltage source 271 isremoved and the transistor 274 is biased “off”, which causes a lightemitting diode 282 to illuminate, thereby indicating that the anti-tipwheels have reached the full up position. The biasing of transistor 274“off” results in the source 278 biasing the second transistor 284 “on”via a 10 K resistor 286. When the transistor 284 is thereby biased “on”,both the high at the first input to the exclusive NOR gate 206 and thehigh output from the inverters 234 and 236 (via diode 288) are drawn offby the transistor 284. The up switch 202 is thereby disabled by openingrelay K1 (element 242) and closing relay K2 (element 226), thus turningoff the motor 50.

The down-limit switch 290 works in a similar manner as the up-limitswitch 272. The down-limit switch 290 is connected to a 5 volt source289 and is normally closed, connecting the source 289 to a transistor292 via a 1 K resistor 294. The connection between the transistor 292and resistor 294 is grounded via a resistor 295. While the down-limitswitch 290 is closed, transistor 292 is biased “on”, draining another 5volt source 296 connected thereto through a 1 K resistor 298, andkeeping a second transistor 302 unbiased and “off”. The down-limitswitch 290 is opened when the anti-tip wheels travel to their lowestallowed point near the ground. The down-limit switch 290 can be openedmechanically by the wheel arm 24, or via a relay connected to a remotesensing device (not shown). When the down-limit switch 290 is open, theconnection to the voltage source 289 is removed and the transistor 292is biased “off”, which causes a light emitting diode 300 to illuminate,thereby indicating that the anti-tip wheels have reached the full downposition. The biasing of transistor 292 “off” results in the voltagesource 296 biasing the transistor 302 “on” via a 10 K resistor 304. Whenthe transistor 284 is biased “on”, both the high at the second input tothe exclusive NOR gate 206 and the high output from the inverters 256,258 (via diode 306) are drawn off by the transistor 302, therebydisabling the down switch 204 by opening the relay K3 (element 264),closing the relay K1, and thus turning off the motor 50.

The up and down switches 202, 204 should be easily accessible by theuser. One place where the motor 50 control switches may be mounted is onthe arm 70 near control stick 72. The motor 50 may also be connected toa secondary control unit which cooperates with a proximity or impactsensor. When so connected, the impact or proximity sensor automaticallyactivates the up circuit when an obstacle in the path of travel isdetected, driving the motor 50 in the upward direction to disengage theretaining mechanism (by rotating the cams 56, as discussed below), andallowing the anti-tip wheels to rise out of the near ground position sothat the wheelchair can clear the obstacle.

The operation of the lowering mechanism of the first embodiment is bestshown in FIGS. 3 a and 3 b. When the cams 56 are in the up position,anti-tip wheels 22 and arm 24 are in a neutral position, being tendedthere by the spring 40. As a user supplies power to the motor 50, motor50 provides rotational motion to gearbox 52. Gearbox 52 transfers inputpower to rotational power in the cam shaft 54. As cam shaft 54 rotates,cams 56 are rotated into their downward position, engaging followers 36.Followers 36 and arm 24 are forced downwardly against the tendency ofspring 40. Wheels 22 are lowered along with arm 24. When wheels 22 reachthe desired position near the ground, the motor 50 stops providing powerto the system. Cams 56 are thereby held in a downward position, engagingthe wheels into the near ground position. When a user wishes to releasethe wheels 22 from the near ground position or when an attachedproximity or impact sensor provides an appropriate signal, the motor 50is supplied with power again. The motor provides power to the gearbox 52which rotates the cam shaft 54. The cams 56 are thereby rotated backinto their upward position. With cams 56 disengaged from followers 36,spring 40 tends back to its relaxed state, pulling arm 24 and wheels 22up into the neutral position.

A second embodiment of the invention, shown in FIGS. 4 through 6, may beused in cooperation with a wheelchair suspension system, and iscurrently preferred. An exemplary suspension system is disclosed by U.S.Pat. No. 6,129,165 to Schaffner, et al, incorporated herein byreference.

The wheelchair of this embodiment, shown in FIGS. 4 a through 4 c anddescribed more fully in the '165 patent, includes a frame 102 having eyemembers 104 and a supporting assembly 106 rigidly attached thereto. Eachof a pair of drive wheels 108 is rotatably associated with anindependent suspension member 110 at its center of rotation 112. Thesuspension member 110 is pivotally attached to the frame 102 via eyemember 104 at a pivot connection 114. A coil spring 116 is wrappedaround a shaft 118 and at one end preferably abuts web 120.Alternatively, the lower end of spring 116 may be fixedly connected toshaft 118 or to a sleeve fitted about an unnumbered shaft which extendstransversely between web 120 and a web which is hidden from view inFIGS. 4 a through 4 c.

An anti-tip wheel arm 122 is fixedly attached to suspension member 110.The arm 122 preferably includes a pivotal adjustment mechanism (notshown) for adjusting the height of an anti-tip wheel 124. Such amechanism is described in U.S. patent application Ser. No. 09/765,022,filed Jan. 18, 2001 and incorporated herein by reference.

FIG. 5 shows the arm 122 associated with supporting assembly 106 bystrut 126 and locking rod 128. A spring 130 is coiled about strut 126.The supporting assembly 106 is fixedly attached to the frame 102, andincludes one or more support arms 132 extending from the frame 102, astrut bracket 134, a locking rod bracket 136, and a solenoid bracket138.

The locking rod 128 is slidingly engaged with the locking rod bracket136 through an aperture therein. The locking rod bracket 136 rotatablysupports a gear shaft 140 and a gear 142, which may be freely engaged ordisengaged with gear shaft 140 as described below.

A ratchet 144 is similarly associated with gear shaft 140 such that ittoo can be engaged or disengaged therefrom. The ratchet 144 is engagedwith solenoid bracket 138 (or elsewhere on the supporting assembly 106)such that gear shaft 140 is limited to one direction of rotationalmovement when ratchet 144 is engaged therewith.

A solenoid 146 is attached to solenoid bracket 138 and is rotatablyconnected to gear shaft 140. Actuation of the solenoid 146 causes thegear shaft to slide back and forth within its engagements with thesupporting assembly, slidable engagement points being provided in thearms 132 and locking rod bracket 136.

Shown in FIG. 6, the gear 142 engages teeth 148 provided on locking rod128. The gear 142 and ratchet 144 are each provided with an aperture150, 152 which has a non-round shape. The shape of apertures 150, 152may be square, or any number of other shapes which would not allowrotational slippage. The shape of apertures 150, 152 may be a circlewith a notch protruding therefrom, as shown in the Figures. The gearshaft 140 is provided with regions having a shape corresponding to theshape of apertures 150, 152. In the embodiment shown in FIG. 6, gearshaft 140 is provided with tabs 154, 156. Tabs 154, 156 may beassociated with the notch in each of apertures 150, 152 such that gear142 and ratchet 144 rotate in unison with gear shaft 140. However,actuation of the solenoid 146 causes gear shaft 140 to slide out ofengagement with the gear 142 and ratchet 144, allowing gear 142 torotate independently of gear shaft 140. Alternatively, the gear 142,ratchet 144 and gear shaft 140 system may be constructed such that thegear shaft 140 remains engaged with either gear 142 or ratchet 144notwithstanding solenoid actuation, but not both. Accordingly, thedesign shown in FIG. 6 may be modified by extending the length of eithertab 154 or 156 so that gear 142 or ratchet 144 remains in rotationalunison with gear shaft 140 at all times. Any of these alternativesprovides an acceptable retaining system 158.

It is, of course, understood that other means, such as a systememploying bearings, can be used in the second embodiment in place of theratchet system described herein. However constructed, the retainingsystem must allow lowering of the anti-tip wheels and must hold theanti-tip wheels in the near ground position until released.

While the retaining system 158 is engaged, (i.e. when both the gear 142and ratchet 144 are in rotational unison with gear shaft 140 and thegear 142 is engaged with the teeth 148 of locking rod 128), a loweringmovement of arm 124 pulls downwardly on strut 126 and locking rod 128,causing rotation of gear 142 and gear shaft 140. The downward movementof locking rod 128 causes gear shaft 140 rotation in the directionpermitted by ratchet 144. When the arm 122 is subjected to an upwardforce, which is in turn transferred through locking rod 128 and gear 142to gear shaft 140, the upward force tends to rotate the gear shaft 140in the opposite direction in which it was rotated during the downwardmovement of arm 124. However, the ratchet 144 prevents rotation in thatdirection, thereby locking down the arm 124.

The wheelchair of this embodiment is equipped with one or more releasetrigger devices (not shown) for actuation of the solenoid 146 whichreleases the retaining system 158. The release trigger may be a switch(not shown) mounted on the wheelchair armrest and connected to a powersupply. Preferably, the release trigger comprises a proximity sensor oran impact detector (neither shown) for sensing an obstacle in the pathof travel. A proximity sensor, such as the type described in U.S. Pat.No. 5,701,122 to Canedy, incorporated herein by reference, may use oneor more optical sensors to detect objects within a predetermineddistance in front of the anti-tip wheels. A mechanical impact detector,such as the type described in U.S. Pat. No. 5,485,140 to Bussin, alsoincorporated herein by reference, may be positioned on the anti-tipwheel assembly such that an impact probe attached to a flexible rodextends forward thereof. The proximity sensor or impact detector canprovide a signal for actuating the solenoid 146, thereby releasing theretaining system 158.

Returning now to FIGS. 4 a through 4 c, the function of the secondembodiment of the invention will be described. Whether the retainingsystem is engaged or disengaged, the anti-tip wheels 124 tend tomaintain a neutral position above the ground when free of influence fromthe lowering mechanism and free of external upward or downward forces,as shown in FIG. 4 a.

The lowering mechanism in this embodiment involves the suspensionsystem. The anti-tip wheels 124 are forced downwardly in response to adeceleration force, such as when the wheelchair is brought to a stop ona smooth surface, when negotiating an incline, or when a drop edge isencountered in the path of travel. FIG. 4 b shows a power wheelchairapproaching a drop edge. As is more fully described in the '165 patent,the occupant of power wheelchair reduces output torque of the drivemotors (not shown) creating a load on drive wheels 108 from theassociated drive motors. This tends to decelerate the drive wheelsrespecting rotation in the direction indicated by arrow AA in FIG. 4 b.This deceleration of drive wheels 108 causes suspension member 110 torotate about pivot point 112 in the direction of arrow AA in FIG. 4 b,causing anti-tip wheels 124 to move downwardly. The rotation ofsuspension member 110 compresses spring 116. Once the decelerationeffect has ceased, spring 116 tends to raise the anti-tip wheels 124back into the neutral position. However, with the retaining system 158engaged, locking rod 128 prevents the wheels from rising. Thus, theanti-tip wheels 124 are locked in the near ground position, preventingany substantial tipping of the wheelchair.

The operator may release the wheels by controlling the solenoid 146 ifdesired. Otherwise, the anti-tip wheels 124 may be left in the nearground position until the wheelchair encounters an obstacle in itstravel path. Preferably, when the obstacle is encountered, the releasetrigger device detects the object and sends a signal to automaticallyactuate the solenoid. As noted above, actuation of the solenoiddisengages the retaining system and allows the force exerted by spring116 to rotate the suspension member 110 back into the neutral position,thereby raising anti-tip wheels 124 to the neutral position.

While the retaining system is disengaged, the anti-tip wheels 124 may beforced upwardly by an upward force supplied by the encountered obstacle160, as shown in FIG. 4 c. When this occurs, spring 130 absorbs theshock of the impact, and allows the anti-tip wheels 124 to rise smoothlyuntil adequate clearance for climbing over the obstacle is achieved.

A number of variations of this embodiment should be apparent to oneskilled in the art from the description provided herein. Rather than asystem involving ratchet 144 or bearings (discussed above), theretaining system 158 may comprise a locking mechanism (not shown)provided to secure gear 142 and gear shaft 140. The locking mechanismmay be any known device which would prevent the gear 142 and gear shaft140 from turning. Such devices could include a pivotally attached tab orhook on the frame, or a cord, wire or the like. Other devices such asbrakes and clamps should also be apparent to those skilled in the art.The user may engage the locking mechanism to prevent the gear 142 fromspinning under the tendency of spring 116 while the anti-tip wheels arein the near ground position, thereby holding the arm 122 and wheels 124in the near ground position. Alternatively, the locking mechanism mayact on the locking rod 128 directly. Such a locking mechanism ispreferably controlled by sensors providing signals indicative of theanti-tip wheels height off the ground. The locking mechanism may bequickly disengaged electronically via a signal from an operator switchor automatically from a release trigger system such as one describedabove.

Further variations are within the scope of the invention, some of whichmay be achieved by combining elements of the two representativeembodiments shown in the Figures. For example, a wheelchair may includean anti-tip wheel arm fixed to an independent suspension member andresponsive to changes in rotational velocity of the associated drivewheel as described with regard to FIGS. 4A through 4C, and a motordriven cam system as shown in FIGS. 3A through C. Such a wheelchair mayalso include the anti-tip wheel control circuit described with referenceto FIG. 7.

It is to be understood that other similar embodiments may be used, ormodifications and additions may be made to the described embodiments forperforming the same function of the present invention without deviatingtherefrom. Therefore, the present invention is not limited to the twoembodiments shown, but rather should be construed in breadth and scopein accordance with the recitation of the appended claims.

1. A power wheelchair comprising: a frame; a seat mounted on the frame;a motor for controlling and driving at least one of a pair of drivewheels; an anti-tip wheel assembly pivotally attached to the frame andsupporting one or more anti-tip wheels, the anti-tip wheel assemblyresiliently positioning the one or more anti-tip wheels in apredetermined neutral position relative to the ground on which the drivewheels normally ride; and a retaining system holding the one or moreanti-tip wheels in a fixed position relative to the ground, theretaining system having intermeshing teeth that selectively engage andrelease.
 2. A power wheelchair comprising: a frame; a pair of drivewheels operatively positioned on opposite sides of the frame; a seatmounted on the frame; a drive motor for driving at least one of a pairof drive wheels; an anti-tip wheel mounted to a suspension member, thesuspension member pivotally supported on the frame and having aresilient suspension; and a retaining system for selectively holding theanti-tip wheels in a fixed position relative to the ground on which thedrive wheels normally ride.
 3. The power wheelchair of claim 2 furthercomprising a release trigger device adapted to disengage the retainingsystem.
 4. The power wheelchair of claim 3 wherein the release triggerdevice automatically sends a signal for releasing the retaining systemin response to detection of an obstacle in the path of travel of thewheelchair.
 5. The power wheelchair of claim wherein the retainingsystem comprises a solenoid, actuation of the solenoid causes engagementand disengagement of a series of intermeshing teeth that fix theposition of the suspension arm with respect to the frame.
 6. The powerwheelchair of claim 5 wherein a solenoid is rotatably attached to a gearshaft having a series of teeth thereon, and the selective actuation ofthe solenoid causes engagement and disengagement of the gear teeth witha ratchet.
 7. A power wheelchair comprising: a frame; a seat mounted onthe frame; a pair of drive wheels, at least one pitch stabilizing wheel;a motor for controlling and driving each of said drive wheels; asuspension system pivotally mounting the pitch stabilizing wheel to saidframe and enabling resilient translation of said pitch stabilizing wheelrelative to said frame, and a wheel restraint having intermeshing teethfor operatively locking pivotal motion of the pitch stabilizing wheelrelative to said frame.
 8. The power wheelchair according to claim 7wherein said suspension system is operatively couple to the motor and ispivotally responsive to the torque generated by the motor and applied tothe corresponding drive wheel.
 9. The power wheelchair according toclaim 7 wherein the pitch stabilizing wheel is normally positioned offthe ground and the wheel restraint is operative to lock said pitchstabilizing wheel at a near ground position.
 10. The power wheelchairaccording to claim 7 wherein said wheel restraint comprises a ratchethaving the plurality of teeth and at least one engagement tooth disposedin combination with the suspension system for operatively engaging theteeth of said ratchet, said ratchet and engagement teeth beingconfigured to provide unidirectional retention such that said wheelsuspension system is, upon activation, locked in a fixed pivotalposition relative to the frame.
 11. The power wheelchair according toclaim 7 wherein said wheel restraint employs a ratchet having theplurality of teeth, said ratchet disposed in combination with saidsuspension system, and at least one tooth disposed in combination withsaid frame and engaging said teeth of said suspension system ratchet,said teeth being configured to provide unidirectional retention suchthat said wheel is selectively locked at fixed position in response tothe torque generated by the motor.